In the manufacture of semiconductors, layers of insulation are formed by chemical vapor deposition (CVD) on thin substrates or wafers, of semiconductive material, such as single-crystal silicon. Certain gases, such as a vapor of an organic liquid and ozone, when mixed together react and deposit insulating material, such as thin layers of silicon dioxide, on exposed surfaces of the semiconductive wafers. Residues of the reactive gases are also deposited on the walls and other surfaces within a chamber in which the wafers are being processed. It becomes necessary therefore periodically to clean the walls and surfaces of the chamber of such accumulated chemical residues since they would otherwise degrade manufacturing quality levels, and possibly affect performance of the semiconductors being produced.
One system currently in use for cleaning a wafer-processing chamber of unwanted chemical residues employs a capacitive radio-frequency (RF) in-situ plasma clean. Disadvantages with such a system include the emissions of perfluoro-compounds (PFCs) that increase global warming through the greenhouse effect, possible damage to critical chamber parts by the RF plasma, and a relatively slow clean time which in turn slows production throughput of semiconductor parts.
Another cleaning technique presently employed uses microwave plasma dissociation of cleaning gas at a location remote from the wafer-chamber. A gas, such as NF.sub.3, is passed through a short tube, or applicator, made of a material such as alumina transparent to microwave energy. The tube is mounted within the housing of a microwave resonator. Gas flowing through the tube is dissociated in a hot plasma within the tube when microwave energy is applied to the resonator, and this forms highly reactive species that react with deposition residue within the wafer processing chamber located downstream of the remote plasma source. The plasma tube at its ends, where it enters and leaves the microwave resonator, is typically sealed by elastomeric O-rings to prevent atmospheric leakage into the tube.
The ionized gas in the plasma tube becomes extremely hot (e.g., over 300.degree. C.) and some of this heat is transferred to the wall of the tube. Unless effective steps are taken to prevent it, heat damage, chemical attack accelerated by elevated temperature, or melting of the O-rings at the ends of the plasma tube will take place. This requires that the equipment be shut down, purged of gas, and new O-rings put in place. This is expensive in lost production time as well as in parts and labor in replacing the O-rings.
Previously, with such microwave ionizing apparatus cooling of portions of the apparatus in order to protect the O-rings was effected by liquid flowing in metal tubes placed against or within the apparatus at suitable places and serving as heat sinks. The complexity of this system adds considerably to the overall cost of the apparatus. It is desirable therefore to have a much simpler cooling system. Cooling by forced air alone has been tried, but is only effective for low power plasma sources that cannot supply enough reactive species to be commercially viable.
It is desirable to have a simple, inexpensive, yet highly effective solution to the mechanical and thermal problems of protecting the O-rings from excessive heat and consequent damage.